Page 429 - Marine Structural Design
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Chapter 22 Material Selections and Damage Tolerance Criteria          405

                  expert judgement  should be  used  to  quantify the  fatigue strength improvement due to  the
                  modification of the weld profile.
                  22.3.4  Modification of Residual Stress Distribution
                  By using the following methods, the undesirable tensile residual stresses found at the weld can
                  be modified to obtain desirable compressive stresses at the weld toe:

                  Stress Relief
                  Various fatigue tests on simple small plate specimens indicate that improved fatigue strength can
                  be  obtained by  stress  relief  due  to  post-weld heat  treatment  (PWHT).  However, plate  and
                  stiffening elements of continuous systems rarely require stress relief.  It is also doubtful that a
                  complex structural detail with built in constraints can be effectively stress relieved.
                  Compressive Overstressing
                  Compressive overstressing is a technique in which compressive residual stresses are introduced
                  at the weld toe. Experimental results and analytical work demonstrate the effectiveness of pre-
                  overstressing, but  the procedure to be  implemented  does not  appear to be practical  for most
                 marine structures.

                  Peening
                  Peening is a cold working process intended to produce surface deformations with the purpose of
                 developing residual compressive stresses.  When  impact  loads on the material surface would
                 cause the surface layer to expand laterally, the layer underneath prevents such surface layer
                 expansion, creating the compressive residual stresses at the surface. Typical peening methods
                  are hammer peening, shot peening, and needle peening.
                  22.3.5  Discussions
                 Fatigue strength improvement techniques are time consuming and costly and they should be
                 applied selectively.  Comparison of different techniques allows assessment of their effectiveness
                 and cost.  The recommended improvement strategies depend on the characteristics of the (global
                 and local) structure and the preference for one technique over others is based on effectiveness,
                 cost and fabrication yard characteristics.
                 Some of the comparisons of various approaches available, which  improve fatigue strength of
                 welded details, are the following:
                    Full profile burr grinding is preferable to toe burr grinding or disc grinding only, because it
                    results in higher fatigue strength even at a substantial cost penalty.
                    Disc  grinding  requires  the  least  time  and  cost.  However,  it  produces  score  marks
                    perpendicular to the principal stress direction, making this technique less effective than
                    others.
                    Using a high-pressure abrasive water jet (AWJ) process for controlled erosion of the weld
                    toe area can be as effective as grinding.  Its simplicity, speed and non-utilization of heat
                    make controlled erosion very promising.
                    A wider weld pool makes plasma dressing less sensitive to the position of the electrode
                    relative to  the weld  toe, compared with TIG dressing.  Therefore, the  fatigue strength
                    improvement obtained from plasma dressing is better than that obtained from TIG dressing.
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